The present invention relates to inert anodes useful for the electrolytic production of aluminum, and more particularly relates to stable inert anodes comprising a single-phase oxide of nickel and iron.
The energy and cost efficiency of aluminum smelting can be significantly reduced with the use of inert, non-consumable and dimensionally stable anodes. Replacement of traditional carbon anodes with inert anodes should allow a highly productive cell design to be utilized, thereby reducing capital costs. Significant environmental benefits are also possible because inert anodes produce no CO2 or CF4 emissions. Some examples of inert anode compositions are provided in U.S. Pat. Nos. 4,374,050, 4,374,761, 4,399,008, 4,455,211, 4,582,585, 4,584,172, 4,620,905, 5,794,112, 5,865,980, 6,126,799, 6,217,739, 6,372,119, 6,416,649, 6,423,204 and 6,423,195, assigned to the assignee of the present application. These patents are incorporated herein by reference.
A significant challenge to the commercialization of inert anode technology is the anode material. Researchers have been searching for suitable inert anode materials since the early years of the Hall-Heroult process. The anode material must satisfy a number of very difficult conditions. For example, the material must not react with or dissolve to any significant extent in the cryolite electrolyte. It must not enter into unwanted reactions with oxygen or corrode in an oxygen-containing atmosphere. It should be thermally stable at temperatures of about 1,000xc2x0 C., and should have good mechanical strength. Furthermore, the anode material must have sufficient electrical conductivity at the smelting cell operating temperatures, e.g., about 900-1,000xc2x0 C., so that the voltage drop at the anode is low and stable during anode service life.
An aspect of the present invention is to provide an inert anode for use in an electrolytic aluminum production cell, the inert anode comprising a ceramic material consisting essentially of an oxide of nickel and iron which is a single phase at the operation temperature of the electrolytic aluminum production cell. The single-phase ceramic material preferably has a Ni/(Ni+Fe) mole ratio of less than 0.333, for example, from 0.290 to 0.330.
Another aspect of the present invention is to provide an electrolytic aluminum production cell comprising a molten salt bath comprising an electrolyte and aluminum oxide, a cathode, and an inert anode comprising a ceramic material consisting essentially of an oxide of nickel and iron. The ceramic material is a single phase at the operation temperature of the electrolytic aluminum production cell.
A further aspect of the present invention is to provide a method of making an inert anode. The method includes the steps of mixing nickel oxide and iron oxide in a Ni/(Ni+Fe) mole ratio of from 0.290 to 0.330, and consolidating the mixture to form a ceramic material consisting essentially of an oxide of nickel and iron having said Ni/(Ni+Fe) mole ratio, wherein the ceramic material is a single phase at an operation temperature of the electrolytic aluminum production cell.
Another aspect of the present invention is to provide a method of making commercial purity aluminum. The method includes the steps of passing current through an inert anode and a cathode through a bath comprising an electrolyte and aluminum oxide, and recovering aluminum comprising a maximum of 0.2 weight percent Fe and a maximum of 0.034 weight percent Ni. The inert anode comprises a ceramic material consisting essentially of an oxide of nickel and iron which is a single phase at an operation temperature of the electrolytic aluminum production cell.
These and other aspects of the present invention will be more apparent from the following description.